What causes fixed-pattern
noise?

Unlike visible light cameras, the pixels used in thermal
cameras can vary widely in response to neighboring pixels, and some
form of calibration is necessary to normalize the image for best
quality. Sometimes it is not possible to completely remove artifacts in
a thermal imaging system.

One source of fixed-pattern noise can be the non-uniformity
compensation (NUC) terms that are applied to pixels in the array. NUC
is a process whereby a series of uniform but noisy frames are averaged
for the calculation of an offset correction table. The resulting
offsets are applied to each pixel in the array to adjust their response
with respect to other pixels. If the number of frames is not enough to
average out the noise present in the video, then some residual noise is
`burned into` the table. This is why the pattern does not move, and is
called fixed-pattern.